Variable gain amplifier



Dec. 3, 1935. c. N. NEBEL 2,022,972

VARIABLE GAIN AMPLIFIER Filed Aug. 21, 1954 our 22 24 Jl'l'lh lNVENTOR By CNNFBEL A TTORNE Patented Dec. 3, 1935 PATENT" OFFICE 2.022.222 VARIABLE GAIN AMPLIFIER Charles N. Nebel, Denville, N. 1., assignor a Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation or New York Application August 21, 1934, Serial No. 740,751

6 Claims.

The present invention relates to amplification of electrical waves, especially of waves comprising a band of frequencies, with provision for varying the amount of the amplification.

In amplifiers generally, and particularly where waves of a band of frequencies are to be amplified, the amplifier characteristic is dependent upon a number of factors entering into the circuit design. Consequently a change made in some circuit element in order tocontrol the gain is liable to resuit in changing the characteristic in some undesired manner.

It is a general object of the present invention to enable the gain of an amplifier to be varied at will within practical limits while maintaining desired amplifier characteristics at the diiierent gain adjustments.

Other objects and the various features and asspects of the invention will be apparent from the following detailed disclosure of a specific embodiment as illustrated in the attached drawing in which Figs. 1 and 2 show alternative forms of embodiment of the invention in amplifier circuits.

In the drawing, in each figure, a single stage of amplification is shown between the input circuit I0 and the output circuit H, although it will be apparent that a plurality of stages of generally similar type may be used in tandem.

The amplifier tube I2, Fig. 1, is shown as a screen grid tube known as' the Western Electric Company No. 259-A, although any other suitable type can be used. The grid circuit and plate circuit are respectively connected to the input and output circuits by transformers I 3 and I4. The

cathode is heated by passing current from a suitable source (not shown) through the heater element shown.

, Anode potential, screen potential and grid bias are obtained from the 130 volt source l5, shown specifically as a battery, the screen lead including a resistor iii of 100,000 ohms and the cathode lead including resistor ll of 1,000 ohms from which the grid bias is derived.

Shunting a variable part of the resistor II, in

accordance with the invention, is a four microiarad condenser i 8 shown connected toresistor I! by a variable tap IQ, for important purposes to be described later on.

Various elements are shown in the circuit for enabling the desired characteristic to be realized.

A 600,000 ohm resistance 20 is connected in shunt of the secondary-input coil, which has a step-up impedance ratio of 600 to 600,000 ohms. Shunting the output primary is a branch comprising a 60,000 ohm resistance 2|, a parallel combination of condenser 22 (.0159 microiarad), inductance 23 (0.152 microhenry) and resistance 24 (700 ohms) and series condenser 25 (.020 microfarad). The output coil works between 60,000 ohms and 600 ohms output circuit. These various circuit ele- 5 ments result in a gain that is fiat within $0.015 decibel from 200 to 3400 cycles, the maximum gain being 37 decibels.

The variable tap on resistor ll provides a simple and eflective gain control which offers several 10 advantages. The maximum gain (37 decibels) is obtained when condenser l8 shunts the whole of resistor i1. As the tap I9 is moved downward (in the figure) along resistor l1, however, a drop of potential at the signal frequencies is developed in 15 the unshunted portion of resistance I! and is applied to the grid circuit in such manner as to lower the amplifier gain In the example given, the gain may readily be reduced in this way byas much as 8 to 10 decibels. 20

By this method of gain control, the amplifier characteristic remains substantially unafiected. It is substantially flat between the limiting frequencies given at all settings of slider l9.

It will be noted that this method of gain control 25 does not affect grid bias or normal plate current or amplifier impedance, but rather affects directly the resultant signal voltages. Thus, sudden changes in the gain adjustment do not set up clicks such as are caused by changes in normal 30 plate current.

As the gain is reduced by the method described, the distortion is decreased as is also any noise originating in the amplifier circuit. This is because the reduction in the gain is produced by 35 a negative feedback action whereby new frequency components first appearing in the amplifier output are reduced in magnitude. Also as the gain is reduced by this method, the amplifier gain is stabilized with respect tochanging 40 battery or other voltages.

The general method of negative feedback for improving amplifier operation is the invention or an entirely different type of coupling may be used for input and output circuits, as desired.

The main point of diil'erence over Fig. 1 lies in the use of a divided resistance iii, 32 through which the space current flows for bias purposes. As the arm 33 is rotated, the shunting condenser I8 is made to shunt a variable part of the total bias resistance as in the case of Fig. 1 but the total bias resistance remains unchanged. This is true because the arm 33 adds as much resistance to the circuit on one side as it subtracts from the other, keeping the total a constant.

One advantage of the type of controller shown in Fig. 2 is that the space current fiows through the variable contacts of the resistance 3|, 32, which results in a quieter circuit in some circumstances. If a high resistance develops at a contact point because of oxidation, dirt or (lust, for example, a high voltage builds up across the contact, which may amount to the full voltage of battery l5, and this voltage tends to break down the film and restore the circuit conditions to normal. If a high resistance develops at the contact IQ of Fig. 1 the efiect is to add this resistance to the shunt resistance of condenser I 8 and this may change the gain or the characteristic or introduce noise since the contact resistance may be variable.

In the foregoing description specific information has been given of a preferred embodiment. It is to be understood that these data are given by way of example in order readily to enable one skilled in the art to make and use the invention, but that the magnitudes of quantities given in no wise limit the invention, which is defined in the claims. The type of tube, type of circuit, frequency range and other matters are capable of wide variation within the invention.

What is claimed is:

1. In an amplifier circuit, a space discharge device having anode and grid or impedance control circuits, resistance common to the grid and anode circuits for applying a negative bias potential to the grid, input and output connections for the waves to be impressed on and derived from said device, and means to control the gain of said circuit comprising a path opaque to direct current but of negligibly low impedance to said waves connected in shunt to a variable portion of said resistance.

2. In an amplifier circuit for amplifying waves of a band of frequencies, an amplifier having an input circuit and an output circuit for said waves,

an input or control element, an anode, resistance common to the input and output circuits for impressing on the input or control element, voltage produced in said resistance by flow of output circuit current of both direct current and of ouris rent representing said waves, and means to control the gain of said circuit comprising means to control the voltage produced in said resistance by currents representing said waves while keeping constant the voltage produced therein by fiow o! 10 direct current.

3. In an amplifier circuit for waves to be amplified, a space discharge device having a cathode, an anode and a grid or control element, a source of anode potential having a negative terminallll connected through a resistor to the cathode and a positive terminal connected to the anode, a gridto-cathode connection including said resistor, input and output connections for the waves being amplified, a path of high impedance to direct 20 current but of negligibly low impedance to currents of the frequency 01 the waves being amplified connected in shunt of said resistor, and means to vary the gain comprising means to vary the amount of said resistor that is shunted by said path.

4. In the operation oi an amplifier oi the type in which the grid bias is obtained from drop-oipotential in plate circuit resistance, the method of controlling the gain comprising varying the amount of said resistance that is traversed by waves being amplified while keeping unchanged the amount of said resistance that is traversed by direct current.

5. An amplifier circuit of the type in which the negative grid bias is obtained from resistance common to the grid and anode circuits, which resistance is shunted by a condenser, characterized by means to control the gain of the amplifier without changing the grid bias, comprising means 40 for varying the amount of the bias resistance that is shunted by the condenser.

6. A circuit according to claim 5 in which said bias resistance is made intwo parts with a movable tap on each part, said taps being connected together electrically and mechanically to move together to vary in complementary manner the amount of each part of the resistance that is included in circuit, said condenser being connected from said taps to the negative end of said resistance.

CHARLES N. NEBEL. 

